The invention relates to an electronic switching device for high-frequency signals. The invention is of particular use in the connection between a microwave frequency antenna and an electronic circuit. This circuit comprises for example one or two channels designed to be connected to the antenna. In the case of two channels, a first channel, usually called the Tx channel, uses the antenna in transmit mode and a second channel, usually called the Rx channel, uses the antenna in receive mode.
A device with two access points used as a switch is well known in the literature under the name of SPST for “Single-Pole, Single-Throw” and a device with three access points used as a switch is well known under the name of SPDT for “Single-Pole, Double-Throw”.
For microwave frequency uses, it is known practice to use as a switching element, diodes comprising an undoped zone, called an intrinsic zone, inserted between doped zones, one positive and the other negative. In the rest of the description, this type of diode will be called a PIN diode, with reference to its name in the literature: “Positive Intrinsic Negative diode”. PIN diodes, reverse biased, have a low capacitance and a high breakdown voltage, while, when forward biased, they have a very low resistance, hence their use in microwave switching.
SPST switching devices have been perfected that comprise two PIN diodes, one, called a serial diode, being connected in series between two access points and the other, called a shunt diode, between one of the points and an earth of the device which also comprises means for biasing the diodes, making it possible to define an on state of the device obtained when the serial diode and the shunt diode are in an on state, and an off state of the device obtained when the serial diode and the shunt diode are in an off state. The connection between the two access points is called a serial branch and the connection between the first access point and the earth is called a shunt branch. The serial branch contains the serial diode and the shunt branch contains the shunt diode.
For an SPDT switching device, four PIN diodes are used, two serial diodes and two shunt diodes. These devices have good performance in terms of adaptation, insertion losses and isolation.
Nevertheless, the existence of electrical interference elements, be they intrinsic to the PIN diodes themselves or associated with the installation of the components of the device on a printed circuit, does not make it possible to obtain optimal levels of insertion losses and isolation.
In order to alleviate this problem, a section of transmission line has been placed in series with each shunt diode. This section is adapted according to the wavelength of the switched signal. A section with a length λ/4, or even slightly less than this value, is chosen so that the section associated with the shunt diode has a length equivalent to λ/4. However, the tuning of the transmission line section limits the pass bandwidth of the device because of the length equivalent to λ/4.